Measuring Clear-Air Vertical Motions From Space

Measuring vertical velocity in the atmosphere has long been a challenge due to its small magnitude. Taking advantage of the modulation of free tropospheric relative humidity by vertical motions, we derive analytical relationships that allow us to retrieve vertical motions in clear air from geostationary measurements of brightness temperature in the infrared absorption band of water vapor. The new observations have a resolution of 1 hr and 2 km in time and space, respectively. They capture the variability of mesoscale and large-scale vertical velocity measured during field campaigns. In the mid-troposphere, clear-sky vertical motions are mostly subsiding but highly heterogeneous in space and time. Around organized deep convective systems, strong subsidence (>500 hPa·day⁻¹) is observed within a distance of a few hundred kilometers. In contrast, transient upward motions of up to 100 hPa·day⁻¹ can occur at the mesoscale. Vertical motions in the clear-sky atmosphere appear to be primarily associated with buoyancy and gravity waves at the mesoscale, and with radiative cooling and equatorial waves at larger spatial scales. This new retrieval reveals a rich range of dynamical features that were previously invisible, thus shedding new light on tropical meteorology.